PROJECT SUMMARY ABSTRACT A more universal vaccine against influenza virus infection is urgently needed. However, a major obstacle limiting more effective and durable vaccines against influenza infection stem from the rapidly shifting nature of viral immune dominant epitopes. Further confounding this obstacle are functional differences in the immunological responsiveness to vaccination and susceptibility of individuals to natural infection primed by prior exposure to influenza antigens. This type of immunological imprinting likely explains the wide discordance in effectiveness of current seasonal influenza vaccines. Considering the near ubiquitous exposure of individuals to influenza virus, together with the wide variability in clinical symptoms from asymptomatic to severe infection and increasingly widespread use of seasonal immunization, immunological imprinting to influenza virus is likely initiated during infancy with the first exposure to natural infection or immunization. Importantly, critical knowledge gaps remain regarding how individuals respond to primary influenza exposure in early life, in the context of natural infection or vaccination, and how a lack of pre-existing immunity effects within-host influenza viral diversity. It is also unclear how this first exposure to influenza impacts the subsequent immunological responsiveness to antigenically identical, similar or discordant influenza epitopes. For infants, the impact of vertically transferred maternal immunity, or that acquired postnatally through breastfeeding, on the quality of ensuing influenza specific immune responses remain unclear. To fill these knowledge gaps, ongoing recruitment of a maternal-infant cohort at Cincinnati Children?s Hospital will be expanded, along with parallel efforts in Mexico City. Both sites have ongoing surveillance providing additional cases of symptomatic primary infection. With the proposed enrollment of more than 2000 pregnant women, our two-site cohort is ideally suited for the proposed studies given our established infrastructure of weekly nasal swabs and symptom reporting, scheduled blood collection to detect asymptomatic and symptomatic influenza, maternal and cord blood and milk sample analyses and detailed evaluations of susceptibility and immunological responses to influenza infection and vaccination in mothers and infants. Our overall hypothesis is that primary influenza exposure in early life impacts the magnitude, durability and breadth of immunological memory to an evolving range of influenza virus antigens and this initial imprint will have a profound effect on subsequent influenza exposures. A team of investigators with complementary expertise in pediatric infectious diseases, epidemiology, maternal-infant cohorts, human B and T cell immunology and influenza virology have been assembled to address our hypothesis by investigating the immunological response of infants to primary influenza virus natural infection compared with immunization (Aim 1), compare the immunological response against an initial exposure to influenza virus via natural infection or immunization in infants (Aim 2), and investigating the impact of pre-existing influenza immunity on virus genetic diversity within individuals (Aim 3).